1. Field of the Invention
This invention relates to a water-solubilized resin composition. More specifically, it relates to a water-solubilized resin composition which is suitable for forming by heat treatment a thermally stable polymer of a high degree of polymerization containing an aliphatic imide ring and an aromatic imide ring in the molecular chain, which can be furnished in a high concentration and with a low viscosity, and which possesses good water-solubility; and to a process for producing the composition.
2. Description of the Prior Art
Because of superior thermal stability, chemical resistance and electrical characteristics, imide-type polymers have heretofore found many applications, for example, as films, adhesives, paints and molded articles used at high temperatures. Thermally stable polyimides are obtained generally by polymerizing aromatic tetracarboxylic acid derivatives such as pyromellitic dianhydride, diethyl pyromellitate or pyromellitic acid dihalides, and diamines in organic polar solvents to form soluble high-molecular-weight polyamide acids, shaping them, and then chemically or thermally treating the shaped products to induce intramolecular cyclization.
Polyamide acids, which are polyimide precursors, readily cyclize during storage to form insoluble infusible polyimides. However, since an expensive basic solvent such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide or N,N-dimethylformamide is used in a large quantity, the polymer solution itself is expensive, and a large quantity of solvent is evolved in the process of manufacturing the final products. The evolved solvent tends to pollute the air and water, and poses a problem of environmental pollution.
On the other hand, water-solubilized compositions which ultimately produce polyimide films are already known (for example, as described Japanese Patent Publication No. 38983/71 (corresponding to U.S. Pat. No. 3,663,728)). Conventional water-solubilized compositions mainly relate to water-solubilized compositions of polyamide acids which are precursors of aromatic polyimides. However, water-solubilized compositions of polyamide acids do not possess sufficient solubility, and are greatly affected by chemical changes such as hydrolysis in the subsequent imidization step and a decrease in the degree of polymerization, for example, occurs. This tends to degrade the properties of the coated film, and the films obtained have poor mechanical characteristics such as strength and elongation.
As disclosed in Japanese Patent Application (OPI) No. 76196/75 (corresponding to U.S. Pat. No. 3,925,313), a technique of water-solubilizing a polyimide precursor, which is obtained by reacting 1,2,3,4-butanetetracarboxylic acid as an acid component with a substantially equimolar amount of a diamine in the presence of a water-soluble solvent and water, by converting it to an ammonium salt, has been previously suggested. While this technique is unique, it has not yet offerred a solution to the problem of improving thermal stability and other characteristics.